The present invention relates to a color image forming apparatus applicable to a color printer, a color copier and a color facsimile, etc., and more particularly, to a color image forming apparatus for forming color images by superimposing toner images of a plurality of colors by use of electrophotography, a belt unit and image forming unit of the color image forming apparatus.
As a conventional color image forming apparatus, the one is known which is described in Japanese Laid-open Patent Application No. Hei 8-152812. The structure of relevant parts thereof is shown in FIG. 19 here as an example of conventional image forming apparatuses.
In FIG. 19, a photoconductor 201 is disposed in contact with an intermediate transfer belt 202 and a developer unit 203 containing toner of different colors (yellow, magenta, cyan and black). An exposure device 204 is disposed in a lower part of the figure. The toner image of each color is formed on the photoconductor 201 by a charger 205, signal light 206 from the exposure device 204 and the developer unit 203 of the four colors, and the toner images are transferred color by color onto the intermediate transfer belt 202. The toner remaining on the photoconductor 201 after the transfer is scraped by a photoconductor cleaner 207 which is always pressed against the photoconductor.
The intermediate transfer belt 202 rotates by being supported between a drive shaft 208 and a driven shaft 209 which are rotating. The toner image of each color on the photoconductor 201 is transferred so as to be aligned every time the intermediate transfer belt 202 rotates once, so that a color image in which the toner images of the four colors are superimposed is obtained on the intermediate transfer belt 202. Then, the color image is secondary-transferred onto recording paper 210 by a secondary transfer roller 211, and the toner image is fused by a fuser unit 212, so that a full color image is obtained.
A cleaning blade 214 of a cleaning unit 213 is pressed against the surface of the intermediate transfer belt 202 with the drive shaft 208 as a backup member. After the color image has been transferred onto the recording paper 210, residual toner which remains on the surface of the intermediate transfer belt 202 is scraped by the cleaning blade 124. Thereby, the intermediate transfer belt 202 is cleaned in order to prepare the next image transfer. The cleaning blade 214 of the cleaning unit 213 is structured so as to be separated from the intermediate transfer belt 202 during the above-described color image formation onto the intermediate transfer belt 202.
In the arrangement described in the Japanese Laid-open Patent Application No. Hei 8-152812, in order to prevent a slip on the contact surfaces between the intermediate transfer belt 202 and the drive shaft 208 during the primary transfer when the cleaning blade 214 is not pressed, a resin having high friction factor is applied onto a part or the whole of the contact surfaces.
As a second conventional color image forming apparatus, an arrangement shown in Japanese Laid-open Patent Application No. Hei 7-36246 will be described. FIG. 20 is a side cross-sectional view showing the general structure of the second conventional color image forming apparatus.
In FIG. 20, an intermediate transfer belt unit 301 includes an intermediate transfer belt 302, a first transfer shaft 303, a second transfer shaft 304, a cleaning roller 305, and a waste toner reservoir 306. Color images are superimposed on the transfer belt 302. Image forming units 307Bk, 307Y, 307M and 307C for black, yellow, magenta and cyan, respectively, constitute an image forming unit group 308, and are angularly disposed at the left portion in the body of the apparatus.
The image forming units 307Bk, 307Y, 307M and 307C are mechanically and electrically integrated with the body of the apparatus by being coupled therewith through a non-illustrated inter-coupling member at an image formation position 310 which is opposed to the transfer belt 302. The image forming unit group 308 is structured so as to be rotatable about a generally cylindrical shaft 309 by a drive force of a frame motor. By this rotation, the image forming units 307Bk, 307Y, 307M and 307C are successively placed in the image formation position 310.
A laser exposure device 312 is disposed in a lower part of the body of the apparatus. A laser signal beam 311 is, as shown in the figure, reflected by a mirror 314 in a shaft 309, scans and exposes a photoconductor drum 318 in the image forming unit 307Bk situated at the image formation position 310, and forms an electrostatic latent image. A developer unit 316 forms a black toner image by developing the electrostatic latent image. The toner image is transferred onto the intermediate transfer belt 302. Then, an image forming unit group 308 rotates 90 degrees, so that the yellow image forming unit 307Y is situated at the image formation position 310.
Then, an operation the same as the black image formation process is performed and the yellow toner image is superimposed on the black toner image on the intermediate transfer belt 302. Then, similar operations are performed by use of the magenta and cyan image forming units 307M and 307C, so that the images of the four colors are completed on the intermediate transfer belt 302. Then, the color image is transferred onto recording paper 330 by a transfer roller 319, and lastly, the image is fixed by a fixing unit 320.
In order to obtain highly accurate full color images in a color image forming apparatus, high accuracy is required for the positioning of the images of the four colors. No problem is caused in practical use in the case where the accuracy of positioning of the images of the four colors is not more than 100 .mu.m. An important point for the positioning is that the periodic speed variations of the photoconductor and the intermediate transfer belt are the same among the images of the colors. Reduction of speed variation of the intermediate transfer belt due to separation and contact of the cleaning blade is also important.
Moreover, since toner images of the four colors are successively formed and superimposed, it takes considerable time to form one sheet of image. It is required to improve the throughput of the color image by reducing the time.
In the arrangement shown in FIG. 19, the cleaning blade is pressed after competition of the primary transfer to the intermediate transfer belt. For this reason, it is necessary that the distance from the primary transfer position to the cleaning position be greater than the image length. Consequently, the peripheral length of the intermediate transfer belt increases, and therefore the time required for one rotation increases. For this reason, the throughput of the image is lowered and it is difficult to reduce the overall size of the apparatus.
Moreover, in the Japanese Laid-open Patent Application No. Hei 7-36246 of the second prior art, no mention is made as to an optimum relationship among the following three: the diameter of the photoconductor which is important for positioning and improvement of the throughput; the peripheral length of the intermediate transfer belt serving as a transfer member; and the image length.
Generally, in order to equalize the speed variations due to eccentricity of rotary members such as the photoconductor and the drive shaft among the colors, the phases of eccentricity of the rotary members are made identical with each other for each color by rotating the rotary members an integral number of times every forming of the image of one color. Thereby, a relationship is obtained that the peripheral length of the intermediate transfer belt is an integral multiple of the peripheral length of the photoconductor.
The diameter of the photoconductor has a low degree of freedom of selection compared with the peripheral length of the intermediate transfer belt. A photoconductor of an appropriate diameter is assumed first. An intermediate transfer belt which is used has a peripheral length which is not less than the sum of the length of the image and the length of a non-image formation section necessary for color switching, and that is an integral multiple of the peripheral length of the photoconductor.
For example, first, it is assumed that the diameter of the photoconductor is 29 mm. Top and bottom margins of 5 mm are set in A4-size recording paper (recording paper with a length of 297 mm) so that the image length is 287 mm. When the non-image formation section necessary for color switching is 85 mm and the peripheral speeds of the photoconductor and the intermediate transfer belt are constant, the necessary peripheral length for the intermediate transfer belt is not less than 372 mm. However in order that the peripheral length of the intermediate transfer belt is an integral multiple of the peripheral length, 91.1 mm, of the photoconductor for the purpose of preventing color displacements due to speed variation, it is necessary that the peripheral length be at least five times, i.e. 455.5 mm. If the length of the intermediate transfer belt is decided as mentioned above, the length of the intermediate transfer belt is considerably long compared with the necessary length 372 mm.
For this reason, the size of the apparatus increases and the rotation period of the intermediate transfer belt increases, so that the throughput of the images of the four colors is lowered. For example, when the running speed of the intermediate transfer belt is 100 mm/second, if the length of the intermediate transfer belt increases by 90 mm, the image formation time for the image of one color increases by 0.9 second. Consequently, for the images of the four colors, the image formation time increases by 3.6 seconds.
Particularly, when the intermediate transfer belt is incorporated in the intermediate transfer belt unit, increase in length of the intermediate transfer belt increases the size of the intermediate transfer belt unit and the size of the body of the apparatus. Moreover, when the photoconductor is incorporated in the image forming unit, increase in diameter of the photoconductor increases the image forming unit. Further, when a plurality of photoconductors are used, increase in diameters of the photoconductors noticeably increases the size of the body of the apparatus. For this reason, it is important to minimize the diameter of the photoconductor and the peripheral length of the intermediate transfer belt. Further, the increase in sizes of the units deteriorates ease of use of the units.
Further, when the photoconductor and the intermediate transfer belt are activated or stopped for each color at the time of switching of the photoconductor, the image quality is liable to degrade if the peripheral length of the intermediate transfer belt is the smallest necessary length. The above-mentioned problems will be described with reference to FIG. 21 to FIG. 23.
FIG. 21 is a view schematically showing the speed of the intermediate transfer belt from the end of image formation of one color to the start of image formation of the next color. Normally, in order to superimpose images on the intermediate transfer belt, it is necessary to detect the position of the intermediate transfer belt 302 and transfer images of a plurality of colors to the same position. For this purpose, for example, position detection means as shown in FIG. 22 is used. A detection hole 302A is provided in the intermediate transfer belt 302, and the position of the intermediate transfer belt 302 is detected by a position detection sensor 321. When the position is detected, a belt position detection signal is generated from the position detection sensor 321. The reference position is detected thereby, and exposure is started in accordance with the timing.
When the speed of the intermediate transfer belt 302 is not stable at the time of the position detection as shown by the broken line V1 of FIG. 21, images written in accordance with a fixed timing are shifted among the colors. For this reason, it is necessary that the movement distance to the position detection be longer than an approach distance until the speed of the intermediate transfer belt becomes constant. The movement distance to the position detection is in a trading-off relationship with the movement distance from the end of image formation to the stop.
FIG. 23 shows a stop position at the image rear end on the intermediate transfer belt 302 at the end of image formation of one color. Here, the distance from a transfer point TP to an image rear end EP is the movement distance from the end of image formation to the stop. A rubbing distance BD is a length where the photoconductor 318 rubs against the intermediate transfer belt 302 at the time of switching of the photoconductor 318.
When the movement distance from the end of image transfer to the intermediate transfer belt 302 to the stop of the intermediate transfer belt 302 is set to be short as shown in the dash and dotted line V2 of FIG. 21 in order to secure a sufficient approach distance at the time of activation, the image rear end stops at a stop position S1 in FIG. 23. In this case, the photoconductor rubs against the rear end of the image on the belt at the time of switching of the photoconductor, so that the image is disturbed.
Conversely, when the distance to the stop is set to be long as shown by the chain double-dashed line V3, the detection hole 302A of the intermediate transfer belt 302 stops at a position S2 of FIG. 22, so that the distance from the position detection sensor 320 is shorter than at a normal stop position S3. For this reason, the belt position detection signal is generated before the speed becomes constant at the time of activation of the intermediate transfer belt 302, so that the positions of the images formed by exposure which is started in accordance with a fixed timing after the generation of the belt position detection signal are shifted from one another on the intermediate transfer belt 302.